Couple of interesting notes. To me, anyway.

The Harbor Freight iron I flattened a few days back wasn't concave. It was S-shaped. Had two curves in it, across the iron. Nice work, Harbor Freight. That is a hard act to follow.

Second thing: when I tried flattening the HF iron on the belt grinder, it seemed like there was some danger of abrading the sides more than the middle, just because belt grinders tend to do that. Yesterday I tried the belt grinder on a Stanley iron, and it seemed to work okay. I didn't push it, though. I went very slow, and I didn't have water beside me, so I didn't grind much.